Catalytic converters clean up

The age-old household tip that lemon juice makes for a great cleaning agent has found new use in the garage.

Researchers have found that a simple wash of citric acid can spruce up exhausted catalytic converters in diesel-powered cars, renewing their pollution-busting properties.

In diesel engines, catalytic converters contain a honeycomb of platinum that cleans up exhaust gases by turning poisonous carbon monoxide and unburned hydrocarbons into more benign carbon dioxide. This breaks down molecules that could contribute to smog.

But sulphur in the fuel and phosphorus from anti-wear oil additives can gum up a converter and prevent it from working. Researchers have tried various methods to clean them out in the past, mostly involving strong acids. But while these often do a good job of wiping away the gunk, they also tend to eat away at the valuable platinum.

Now scientists from the Institute of Catalysis and Petrochemistry in Madrid, Spain, have found that a dilute solution of citric acid can wash out the catalyst killers without damaging the platinum. When tested on a simulated stream of exhaust gases, the cleaned-up catalysts were as good as new, the team reports online in Environmental Science and Technology1.

Catalytic comeback

The citric acid - which was produced industrially rather than by squeezing lemons - removed up to 82% of the phosphorus and about 90% of the sulphur from a catalyst that had been used for 48,000 kilometres of driving in a diesel-fuelled car. The wash cycle took six hours at 80 °C.

"Removing sulphur and phosphorus in this way is a very positive step," says Richard Stobart, an automotive engineer at the University of Sussex in Brighton, UK.

The average vehicle runs for roughly 240,000 kilometres, and catalytic converters are supposed to last this course. But some researchers have claimed that up to 90% of catalysts fail before they reach 80,000 kilometres, says Stobart. Regenerating them periodically could help to reduce emission pollution, he adds.

Cars built in the United States already have on-board emissions monitoring, which should alert the driver when the catalyst starts to fail. Similar guidelines are expected to come into force in Europe within the next few years, says Stobart.

At present, many used catalytic converters are recycled to extract the expensive platinum metal, Stobart notes. But this energy-intensive process wastes the rest of the converter. Reactivation would be much more environmentally friendly, he says.

But it may be more expensive. Stobart says that the only people who would find it economically feasible to clean their converters would probably be those with fleets of diesel trucks that can each cover 2 million kilometres in their lifetime. With more stringent emissions monitoring on the horizon, it would make good sense for these engines to get a regular spring clean. "These are big, valuable devices, and replacing them can cost as much as replacing an engine," says Stobart.